Types of blood-treatment apparatus having a blood-treating unit through which the blood of a patient flows are generally familiar. They include for example the known types of apparatus for haemodialysis, haemofiltration and haemodiafiltration. The known types of blood-treatment apparatus can be operated in the single-needle or double-needle mode.
In the double-needle mode, the blood is withdrawn from a blood vessel of the patient via a first needle, is conveyed to the blood-treating unit of the blood-treatment apparatus and is fed back into a blood vessel of the patient via a second needle. Replaceable tubing sets having a blood infeed line and a blood return line to which the two needles are connected are used for the withdrawal and return of the blood. These tubing sets, which are thrown away after use, are also referred to as disposables.
In the single-needle mode, the withdrawal and return of the blood take place via a single needle. During an arterial phase, the blood withdrawn from the patient is stored in a reservoir, before being then fed back from the reservoir to the patient's circulation through the same needle in a venous phase.
A blood-treatment apparatus for the single-needle mode is described in German Application No. DE 10 2007 026 009. This known blood-treatment apparatus has an extra-corporeal blood circuit which has a blood infeed line running to the inlet of a blood-treating unit and a blood return line leading away from the outlet of the blood-treating unit. This extra-corporeal blood circuit having the blood-treating unit need not be part of the blood-treatment apparatus but may be a disposable intended for once-only use which is inserted in the treating unit for the purpose of the blood treatment.
As well as this, the blood-treatment apparatus also has a device for amassing blood and a device for storing gas and in particular air. The devices for amassing blood and for storing gas are each a reservoir which creates a sealed-off volume of space, and are for example respective blood and air chambers of a preset volume.
The device for amassing blood is connected to the device for storing gas, thus enabling air which is expelled from the device for amassing blood to make its way into the device for storing gas in the venous phase when the device for amassing blood is being filled, and air to make its way from the device for storing gas into the device for amassing blood in the venous phase whereby the blood which previously amassed in the arterial phase is expelled from the device for amassing blood. What is achieved in this way is that blood is withdrawn from the patient in the arterial phase and the blood which has been treated by the blood-treating unit is fed back again to the patient in the venous phase.
The connection between the device for amassing blood and the device for storing gas has a first and a second connecting path. The first connecting path connects the device for amassing blood and the device for storing gas in such a way that air expelled from the device for amassing blood is transferred to the device for storing gas in the arterial phase, with a device for interrupting the connection being provided in the first connecting path. The second connecting path connects the device for amassing blood and the device for storing gas in such a way that air which was stored in the device for storing gas is transferred to the device for amassing blood in the venous phase. The second connecting path includes a device for compressing gas, thus enabling the air which was stored in the device for storing gas to be transferred to the device for amassing blood. A preset pressure can be generated in the device for amassing blood by the device for compressing the air both during the arterial phase and during the venous phase. The device for compressing gas may for example take the form of a conventional compressor.
Together with the first and second connecting paths, the device for amassing blood and the device for storing gas form sealed-off volumes of space which no gas is able to enter and from which no gas is able to escape. Only for the purposes of initialisation is air admitted to/released from the sealed-off volume of space.
For trouble-free and safe operation of the blood-treatment apparatus in the single-needle mode, it must be ensured that air is not under any circumstances able to be infused into the patient.
In the known blood-treatment apparatus, the possibility of blood being infused into the patient is intended to be ruled out by setting, in the device for storing gas, a pressure which under no circumstances in operation is higher than the pressure which arises in the device for amassing blood. What is intended to be achieved by this device is that air is only able to make its way from the device for storing gas into the device for amassing blood with the help of the device for compressing gas. If on the other hand there is a failure of the device for compressing gas, such as a leak in the compressor for example, no air is to be able to make its way into the device for amassing blood. This is intended to rule out the possibility of the device for amassing blood unintentionally running dry, as a result of which air might make its way into the patient.
In the known blood-treatment apparatus, an initialisation of the system takes place before the arterial and venous phases are started. To enable the condition to be met that, when there is a low return pressure and a high cardiac output, air is not to be able to make its way from the device for storing gas into the device for amassing blood if there is a failure of the compressor, the system is depressurised to a preset pressure, which is preferably ambient pressure, to initialise the successive arterial and venous phases, at the time of the changeover from the arterial to the venous phase, i.e., at the time of the upper changeover point at which the device for amassing blood is full. In the successive arterial and venous phases, the pressure in the device for storing gas is thus always below the preset pressure, and particularly ambient pressure, to which the sealed-off system has been de-pressurised.
It is found to be a disadvantage that, before the arterial and venous phases are started, the device for amassing blood has to be filled with blood until the filled level has reached the upper changeover point. What is more, it is necessary for a given blood level in the device for amassing blood to be set beforehand.